LOL! Once these clowns are out of QT I'll be looking into a Royal Gramma. I have also seen people that keep a flame angel or a coral beauty in a 29 gal biocube, what do you think of that. I was also looking into te flame angel but wasnt sure about it.
I have a flame angel currently, and he's small, well fed, and doesn't look like he's growing at all. Also he picks at the rocks and sand, but I haven't seen him pick at any corals. I hear it's really a toss-up though.
I currently have a pair of clowns (who I believe to be laying eggs in a place I can't get to or see, which they guard closely, and now the male bites me if I put my hands in the tank

), a flame angel, and just recently added a McCosker Flasher Wrasse. I want to add 2 more female flasher wrasses at some point, and maybe another fish, though that would be the limit.
I'm a big proponent of less fish if you're going for a coral-dominated tank. Skimming, macro algae growth, and frequent waterchanges help, but the easiest way is just a lower bioload.
My first tank I bought used was a 24g nanocube, and it came with 13 fishes, including a small yellow tang. Fortunately the LFS owner near me was superbly knowledgeable and took in the fishes for credit and helped me get on the right path.
I think you could do ~5-6 fishes without too much trouble, especially since you have the skimmer and purigen.